This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-259536, filed Aug. 29, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to a digital key telephone set which acquires a serial signal of frame structure from a key telephone unit, and a digital key telephone system including the digital key telephone set.
2. Description of the Related Art
A digital key telephone system performs data transfer on a data channel (Dch) based on ping-pong transmission between the key telephone unit and a key telephone set. The ping-pong transmission is a system for transmitting data by generally using the key telephone unit as a master device and using the key telephone set as a slave device. The key telephone set extracts data transmitted via the data channel from the key telephone unit by use of a ping-pong transmitting section and then reads the extracted data by use of a CPU.
Data transmitted via Dch is transmitted in the form of a serial signal (serial Dch signal) of a Dch frame as shown in FIG. 1. The Dch frame is a frame of 11-bit length having parity bits P attached before and after Dch data, and having a start bit ST and stop bit SP respectively attached to the head portion and tail portion of the frame. The Dch data is real data of eight bits (one byte) of b0 to b7. The start bit ST is a bit fixed at xe2x80x9c0xe2x80x9d. The stop bit is a bit fixed at xe2x80x9c1xe2x80x9d.
When eleven bits of the same frame are stored in the internal portion of the CPU, the CPU recognizes the eleven bits as a Dch frame and extracts Dch data from the eleven bits.
In this case, however, the CPU must perform an extremely large number of processes such as a lot of timer operations in order to extract Dch data. As a result, this imposes a heavy load on the CPU.
In a transmission path between the key telephone unit and the key telephone set, a B channel for transmitting voice data or the like is provided in addition to Dch. Dch data, Bch data and the like are multiplexed in a time sharing fashion to constitute a transmission frame having a sync frame of 8 kHz period. In the transmission frame, two bits are assigned for each frame in Dch. Therefore, the serial Dch signal extracted by the ping-pong transmitting section takes a state as shown in FIG. 2 and has a transmission capacity of 16 kbps.
However, in the conventional system, in order to attain stabilization of data, the same bit is transmitted in each of the two periods of the transmission frame. Therefore, as shown in FIG. 3, the same bit is repeatedly transmitted four times, and as a result, the transmission rate of Dch becomes 4 kbps which is xc2xc of the transmission capacity.
In the recent system, a high transmission rate is required for Dch in order to attain highly sophisticated functions and it is considered that the serial Dch signal is transmitted at a transmission rate of 16 kbps by utilizing the transmission capacity to the maximum extent. However, a key telephone set which can cope with the transmission rate of 16 kbps cannot be connected to the key telephone unit which has been used so far.
As described above, in the conventional system, since data is fetched in the digital key telephone key telephone set based on the software process by the CPU, there occurs a problem that the load on the CPU due to the above task becomes extremely heavy.
Further, in the conventional system, the data channel can only cope with a single transmission rate, and in a new series digital key telephone key telephone set in which the transmission rate of the data channel is enhanced, there occurs a problem that the compatibility with the conventional system in which the transmission rate of the data channel is low cannot be attained.
This invention has been made in order to solve the above problems and an object of this invention is to provide a digital key telephone set and a digital key telephone system which can alleviate the load of the CPU and cope with transmission rates of a plurality of data channels.
In order to attain the above object, according to one aspect of this invention, there is provided a digital key telephone set used as a key telephone set in a digital key telephone system which transmits a serial signal of a frame structure including a real data and a start bit arranged in the head portion of the real data, from a key telephone unit to the key telephone set via a data channel, comprising a clock generating circuit which selectively generates one of preset clock signals having different rates in response to a specification from the exterior; a start bit detecting circuit which detects the start bit from the serial signal transmitted to generate a detection signal; a counting circuit which counts the number of clocks of the clock signal in response to the detection signal and generates a fetch signal when the counted number is reached to a preset value; and an output circuit which fetches the serial signal at preset fetch timing which is synchronized with the clock signal, and outputs the real data in response to the fetch signal.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.